Elemental sulfur is a commercially important raw material used in the production of sulfuric acid and numerous other chemicals. The global production of elemental sulfur is about 39 million tons per year, of which about 16 million tons per year are produced in North America. A very significant fraction of currently produced elemental sulfur is handled and stored as a liquid, i.e. in its molten form, at least at one point during the production-storage-transportation-consumption sequence of this material. In fact, some sulfur, after being produced in molten form, is never allowed to solidify during transportation and storage, and is consumed in liquid form as it is fed to sulfur burners to be converted to sulfur dioxide and eventually to sulfuric acid.
In light of the commercial importance of liquid sulfur as a chemical commodity, safe and environmentally acceptable methods of molten sulfur storage constitute an important component of sulfur technology. Current industrial practice entails usage of large heated and insulated liquid sulfur storage tanks, usually shaped in the form of large diameter vertical cylinders, constructed from mild steel, and typically ranging in capacity from 1,000 tons to 10,000 tons of elemental sulfur. In order to minimize heat losses, to prevent rain water from entering the tanks, and to minimize sulfur losses, liquid sulfur storage tanks are covered. Further, in order to allow for thermal expansion/contraction of the stored sulfur caused by temperature fluctuations and to prevent the possible build up of inorganic sulfur gases in the vapor space above the molten sulfur, the covers or roofs of storage tanks are equipped with at least one, and typically with two to ten vents. Since molten sulfur is usually sparged with air to prevent the possible build up in the molten sulfur of dissolved inorganic sulfur gases during storage of the molten sulfur, the vent(s) also serve to release from the tank the air which is used for such sparging.
The temperature of molten sulfur in the storage tanks is usually maintained between about 248.degree. F. and about 302.degree. F. In this temperature range the vapor pressure of elemental sulfur is very low namely between about 0.02 and about 0.2 mm Hg. Thus, the air in the vapor space above molten sulfur in sulfur storage tanks contains low concentrations of sulfur vapor, and may also contain minute quantities of elemental sulfur in the form of mist. As such sulfur-containing air passes through and exits the vent(s), its temperature decreases and at least some of its sulfur content condenses and solidifies into particles which are microscopic in size and tend to remain suspended in air, giving rise to what is referred to herein and in the sulfur industry as the formation and emission of particulate sulfur.
The rate of particulate sulfur emissions from a molten sulfur storage tank is primarily dependent on the air ventilation rate, and is also affected by other factors, including the temperature of the stored sulfur, the air sparging rate, the size and geometry of the tank, and the quantity of stored sulfur. Although the level of particulate sulfur emissions from molten sulfur storage tanks is generally very low, environmental concerns regarding such sulfur emissions persist.
There is no prior art known to the applicant which provides methodology for reducing the emission of particulate sulfur from molten sulfur storage tanks. U.S. Pat. No. 4,391,791 to Palm et al discloses a process and apparatus for the removal of elemental sulfur from gaseous streams, such as the tail gas of the Claus process. Claus process gas is formed through catalytic conversion of hydrogen sulfide and sulfur dioxide to water vapor and sulfur vapor in accordance with the following reaction: EQU 2H.sub.2 S+SO.sub.2 .fwdarw.2H.sub.2 O+3S
The resulting mixture of gases contains high concentrations of water vapor and sulfur vapor, as well as unreacted hydrogen sulfide and sulfur dioxide. The reaction is a chemical equilibrium which can be shifted toward the right side by using two or more catalyst beds operated at successively lower temperatures and by condensing sulfur vapor from the gas stream between the catalytic converters.
The process disclosed by Palm et al in their '791 Patent entails passing a gas stream containing vaporized elemental sulfur through two cooling zones, the first of which is maintained at conditions which serve to effect condensation of elemental sulfur without solidification thereof and the second of which is maintained at conditions which serve to effect the solidification of elemental sulfur, and periodically reversing both the direction of the gas flow as well as the conditions of the two cooling zones. The apparatus for removing and recovering vaporized elemental sulfur from a gas stream disclosed in the '791 Patent to Palm et al is a horizontal two-zone condenser of conventional shell and tube heat exchanger design, equipped with means to reverse the flow of gas to be treated therein, means to switch the temperature conditions of the two condenser zones, and means for recovering liquid sulfur from both heat exchange zones.
U.S. Pat. No. 4,526,590 to Palm et al discloses a process for cooling and condensing sulfur and water from Claus process gas. The process comprises passing a Claus process gas stream through a sulfur condenser, then passing the resulting gas stream through a first cooling zone maintained at conditions to effect condensation and solidification of sulfur without the condensation of water and next through a second cooling zone maintained at conditions to effect condensation of water. The first cooling zone of Palm et al's condenser is operated at a temperature below 225.degree. F., typically about 200.degree. F. while the second cooling zone of Palm et al's condenser is operated at a temperature below 125.degree. F., typically about 100.degree. F. U.S. Pat. No. 4,601,330 covers the corresponding apparatus claims; the '330 Patent to Palm et al issued upon a divisional of copending application which issued as U.S. Pat. No. 4,526,590. The apparatus disclosed by Palm et al in their '330 Patent is a horizontal two-zone condenser of conventional shell and tube heat exchanger design with the means and features required in the practice of the process claimed in their '590 Patent.
The processes or systems proposed by Palm et al for condensing sulfur and water vapor from Claus process gas in the three foregoing patents are not suitable for the reduction of particulate sulfur emissions from liquid sulfur storage tanks. This lack of suitability reflects the complexities of the processes and apparatuses claimed by Palm et al, the geometry of their equipment design, and other factors.
It is therefore an object of this invention to provide a simple, convenient and effective method and apparatus for the reduction of particulate sulfur emissions from liquid sulfur storage tanks.
It is another object of this invention to provide a method and apparatus for the reduction of particulate sulfur emissions from liquid sulfur storage tanks, wherein said method and apparatus do not require the use of condensers with two distinct condensation zones.
It is a further object of this invention to provide a method and apparatus for the reduction of particulate sulfur emissions from liquid sulfur storage tanks, wherein said method and apparatus are intrinsically capable of effecting the return to the liquid sulfur storage tank of sulfur collected in the apparatus of this invention, without the use of sulfur pumps and other auxiliary means.
It is still another object of this invention to provide a method for the removal of sulfur from low-volume gas streams containing low concentrations of sulfur vapor, such as gas streams evolved from liquid sulfur storage tanks.